Lecture Objectives:

Lecture Objectives:

• Learn about automatic control

• Use life-cycle cost analysis integrated in eQUEST

Basic purpose of HVAC control

Daily, weekly, and seasonal swings make HVAC control challenging

Highly unsteady-state environment

Provide balance of reasonable comfort at minimum cost and energy

Two distinct actions:1) Switching/Enabling: Manage availability

of plant according to schedule using timers.

2) Regulation: Match plant capacity to demand

Basic Control loopExample: Heat exchanger control

– Modulating (Analog) control

air

water

Cooling coil

(set point temperature)

x

Cooling coil control valve

Position (x)

fluid

Electric (pneumatic) motor

Vfluid = f(x) - linear or exponential function

Volume flow rate

The PID control algorithm

For our example of heating coil:

Proportional Integral Differential

time

Position (x)

constants

e(t) – difference between set point and measured value

d

TTdTKdTT

T

KTTKx d

i

)()()( measuredpointset

measuredpointset measuredpointset

Proportional(how much)

Integral(for how long)

Differential(how fast)

Position of the valve

The control in HVAC system – only PI

dTTT

KTTKx

i

)()( measuredpointset measuredpointset

Proportional Integral

Proportionalaffect the slope

Integralaffect the shape after the first “bump”

Set point

Set point

value

Detail control system simulationMatLAB - Simulink

Control system simulation - take into account HVAC component behavior but focus more on control devices and stability of control scheme

Models integrated in HVAC System simulation Example:

Economizer (fresh air volume flow rate control)

mixing

damper

fresh air

T & RH sensors

recirc. air

Controlled device is damper

- Damper for the air - Valve for the liquids

HVAC Control

Economizer (fresh air volume flow rate control)

mixing

damper

fresh air

T & RH sensors

recirc. air

Controlled device is damper

- Damper for the air - Valve for the liquids

% fresh air

Minimum for ventilation

100%

Economizer – cooling regime

How to control the fresh air volume flow rate?

% fresh air

Minimum for ventilation

100%

If TOA < Tset-point → Supply more fresh air than the minimum required

The question is how much?

Open the damper for the fresh air

and compare the Troom with the Tset-point .

Open till you get the Troom = Tset-point

If you have 100% fresh air and your still need cooling use cooling coil.

What are the priorities: - Control the dampers and then the cooling coils or - Control the valves of cooling coil and then the dampers ?

Defend by SEQUENCE OF OERATION the set of operation which HVAC designer provides to the automatic control engineer

Economizer – cooling regime

Example of SEQUENCE OF OERATIONS:

If TOA < Tset-point open the fresh air damper the maximum position

Then, if Tindoor air < Tset-point start closing the cooling coil valve

If cooling coil valve is closed and T indoor air < Tset-point start closing the damper till you get T indoor air = T set-point

Other variations are possible

Sequence of calculation in energy simulation modeling is different than sequence of operation !

We often assume perfect aromatic control

Example of Sequence of calculation in energy simulation models

HVAC solver calculates Q using

plant_real Matrix solver results

Matrix solver solves newT and T

for Qrad_surf air_real

plant_real

T < Trad_sur f max

Matrix solver solves Q for

T or Tplant_corec

max min

for heating:

or for cooling:T > Trad_surf_corec min

yes

yes

no

no

controlled Trad_surf controlled Tsupply

Matrix S.

for

air_setpoint

solves required T and Q

Trad_surf plant

HVAC solver calculates Q using

plant_real

Matrix solver results

Matrix solver solves new T and T

for Q supply air_real

plant_real

T < T < Tmin supply max

Matrix solver solves Q for

T or Tplant_corect

max min

yes

yes

no

no

controlled msupply

Matrix S.

for

air_setpoint

solves required T and Q

Tsupply plant

Q < Qplant_real plant Q < Qplant_real plant

HVAC solver calculates Q using

plant_real

Matrix solver results

Matrix solver solves new m and T

for Qsupply air_real

plant_real

Matrix solver solves Q for

m or m and Tplant_corect

max min supply

yes

yes

no

no

Matrix S.

for

air_setpoint

solves required m and Q

Tsupply plant

Q < Qplant_real plant

Matrix Solver

for

air_setpoint

solves Q

Tplant_recqured

Matrix solver

for Q

solves T

= 0air

plant_recqured

Matrix solver

for air_setpoint

solves Qplant_recqured

T

HVAC solver calculates Q using

plant_real

Matrix solver results

Matrix solver so lves T

for Qair_real

plant_real

yes

no Q < Qplant_real plant_required

contro lled pure convective source

m < m < mmin supply max

If

corect

zone reheater and m > m

T and check

T < T < T

min supply

supply

min supply max

load calculation HVAC is OFF

HVAC control is ON

II III IV V

VII

Life Cycle Cost Analysis

• Engineering economics

Life Cycle Cost Analysis

• Engineering economics

• Compound-amount factor (f/p)• Present worth factor value (p/f) • Future worth of a uniform series of amount (f/a)• Present worth of a uniform series of amount (p/a)• Gradient present worth factor (GPWF)

Parameters in life cycle cost analysis

Beside energy benefits expressed in $,you should consider:

• First cost• Maintenance• Operation life• Change of the energy cost • Interest (inflation)• Taxes, Discounts, Rebates, other Government

measures

Example

• Using eQUEST analyze the benefits (energy saving and pay back period)

of installing

- low-e double glazed window

- variable frequency drive

Floor heating system

Solar radiation

Floor heating tank

Perforated tube

Floor heating system

P2

T3

T4

Example project

Solar collector system

)]([ OAiLossRSCSC TTUSFAQ

)( iopSC TTmcQ

)()( OAiTANKheatingSCp TTUAQQd

dTmc

TANKUA)(

LossU

Solar collector

Water flow

Water tank

Area

Property of solar collector

Total solar radiation

coefficient which define lost of energy from solar collector surfaces to surrounding

define lost of energy from water tank to surrounding

Used energy